This is a multicenter, randomized, double-blind, placebo-controlled clinical trial to determine whether a recombinant human interferon beta product, inter-feron beta-1a (AVONEX), is beneficial in delaying the onset of a new neuro-logical event in subjects who have experienced a first and recent onset of a demyelinating event involving the optic nerve, spinal cord or brainstem and who are at high risk for multiple sclerosis based on the presence of multiple brain magnetic resonance imaging (MRI) signal abnormalities. Recruitment for this study has closed. Three patients remain in the double-blind study. Several patients have received a clinical diagnosis of Multiple Sclerosis and have thus reached the study end point.